1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of Related Art
A display device using a liquid crystal has been broadly applied to a product that consumes low power and is thin type as one of flat panel displays used in place of a CRT.
A liquid crystal display device (LCD) includes a simple matrix type LCD and a TFT-LCD that uses a thin film transistor (TFT) as a switching element. The TFT-LCD is superior to the CRT or the simple matrix type LCD in terms of portability and display quality, and has been broadly used in a lap top computer and so on. In general, in the TFT-LCD, a liquid crystal layer is disposed between a TFT array substrate having the TFT formed thereon in array and an opposing substrate. Then, a polarizing plate is placed on the outer surfaces of the opposing substrate and the TFT array substrate, and a light source is further provided on one side. By having such a structure, excellent display can be realized in the TFT-LCD.
The TFT-LCD includes a reflective type TFT-LCD that displays images by reflecting a light that is externally incident by a reflector in addition to a transmissive type TFT-LCD that displays images by transmitting a light of a backlight embedded as a light source. Further, there is also a transflective TFT-LCD that uses both the transmissive and reflective methods. In the transflective liquid crystal display device, reflection of daylight is used under bright ambient light, and a backlight is used under dark ambient light, and thus, excellent display characteristics can be obtained under both indoor and outdoor circumstances. In recent years, as mobile display devices have been widely used, there has been a growing demand on the transflective TFT-LCD panel for a small-sized display such as a portable telephone and a portable music player, and a medium-sized display such as a portable video player, a PDA, and an in-vehicle navigation.
In the TFT-LCD, the TFT needs to be formed on a glass substrate to have an array shape using a semiconductor technique in manufacturing the TFT array substrate, which requires large number of processes. As such, large number of devices are needed for the manufacture and the manufacturing cost becomes high. Especially, in the transflective TFT-LCD, both of the reflective pixel electrode and the transmissive pixel electrode need to be formed, which increases the manufacturing cost as larger number of processes are required compared with the general transmissive TFT-LCD or the reflective TFT-LCD.
For example, Japanese Unexamined Patent Application Publication No. 2005-215277 discloses a technique of reducing the number of photomasks used for manufacturing the TFT array substrate of the transflective TFT-LCD. In Japanese Unexamined Patent Application Publication No. 2005-215277, the reflective pixel electrode and the transmissive pixel electrode of the pixel electrode are formed by one photolithography using a halftone exposure technique. Accordingly, the TFT array substrate that is conventionally formed by six photolithography processes can be formed by five photolithography processes in Japanese Unexamined Patent Application Publication No. 2005-215277, which means the number of photomasks can be reduced.
When the reflective pixel electrode and the transmissive pixel electrode are formed by one photolithography using the method disclosed in Japanese Unexamined Patent Application Publication No. 2005-215277, after forming the transparent conductive layer to serve as the transmissive pixel electrode and the reflective metal layer to serve as the reflective pixel electrode, the resist pattern having a difference in film thickness is firstly formed. The reflective metal layer is etched using the resist pattern having a difference in film thickness as a mask. Next, the thin film portion of the resist pattern having a difference in film thickness is removed by oxygen plasma processing. After that, the transparent conductive layer is etched using the reflective metal layer and the resist pattern in which the thin film portion is removed as masks. Then, the reflective metal layer is etched again using the resist pattern in which the thin film portion is removed as a mask. As such, the reflective metal layer of the transmissive pixel portion is removed to form the reflective pixel electrode and the transmissive pixel electrode.
In general, in order to remove the thin film portion of the resist pattern having a difference in film thickness, ashing for oxidatively decomposing the resist by a dry etcher such as oxygen plasma processing, for example, is performed. However, according to the method disclosed in Japanese Unexamined Patent Application Publication No. 2005-215277, the ashing is carried out with a state in which the transparent conductive layer is exposed on the surface, which may cause abnormal discharge. The abnormal discharge causes damage not only to the transparent conductive layer but also to an organic film provided therebelow. Further, failure may be caused such as disconnection of a line provided in a lower layer.
On the other hand, there is also a method of removing the exposed transparent conductive layer in advance before the ashing in order to prevent the abnormal discharge in the ashing. More specifically, the transparent conductive layer and the reflective metal layer are etched using the resist pattern having a difference in film thickness as a mask, followed by the ashing, and then, the reflective metal layer is etched again using the resist pattern where the thin film portion is removed as a mask. As is similar to the method disclosed in Japanese Unexamined Patent Application Publication No. 2005-215277, this method also enables to form the reflective pixel electrode and the transmissive pixel electrode by one photolithography.
However, according to this method, the transparent conductive layer is removed, and thus, the underlayer organic film is exposed on the surface. In the transflective liquid crystal display device, the organic film having a concave and convex pattern on the surface is provided below the pixel electrode in order to obtain excellent scattering characteristics. The thickness of the organic film of the exposed portion is decreased as is similar to the resist pattern due to the ashing to remove the thin film portion of the resist pattern having a difference in film thickness. Accordingly, the film thickness of the organic film covered with the transparent conductive layer and the film thickness of the organic film which is not covered with the transparent conductive layer greatly vary with each other.
A cross sectional view of the related liquid crystal display device using the thus-formed TFT array substrate is shown in FIG. 17. In FIG. 17, a TFT array substrate 10 and an opposing substrate 30 are arranged opposite to each other. Then, a liquid crystal layer 36 is disposed in a space with a sealing material 37 that bonds the both substrates. The sealing material 37 is formed to have a frame shape so as to surround the display region of the liquid crystal display device.
In the TFT array substrate 10, a gate line (not shown) and a source line (not shown) are formed over a substrate with an insulating film (not shown) interposed therebetween. Then, an organic film 18 is provided in the upper layer of the gate line, the source line, and the insulating film. On the organic film 18, a pixel electrode 19 where a transmissive pixel electrode 191 and a reflective pixel electrode 192 are stacked is formed in each pixel. A region where the pixel electrodes 19 are arranged in matrix is a display region 41. The film thickness of the organic film 18 covered with the transparent conductive layer is different from the film thickness of the organic film 18 which is not covered with the transparent conductive layer. Thus, in a part which is not covered with the transparent conductive layer, which means in a region between pixels and a frame region 42, the film thickness of the organic film 18 is thinner than that in the pixel region.
In the opposing substrate 30, a BM 32, a color material 33, and an opposing electrode 34 and so on are formed over a substrate. Then, a columnar spacer 35 to determine the gap with the opposing TFT array substrate 10 is provided on the opposing electrode 34. The columnar spacer 35 is formed in the display region 41 and the frame region 42. More specifically, in the display region 41, the columnar spacer 35 is arranged in a position opposed to the reflective pixel electrode 192. On the other hand, in the frame region 42, the columnar spacer is arranged in a region from outside the display region 41 to inside the sealing material 37. However, the organic film 18 in this part has a smaller film thickness than that of the pixel region as described above. Accordingly, as shown in FIG. 17, it is impossible to keep the gap between the both substrates even, which causes a gap failure. Due to this gap failure, the display failure such as the display unevenness occurs at the periphery of the display region 41 (peripheral gap unevenness), which degrades the display quality of the liquid crystal display device.
In recent years, the glass substrate that is used for both the TFT array substrate 10 and the opposing substrate 30 has been thinner and thinner in order to realize the reduction in thickness and weight required in the liquid crystal panel, which decreases the mechanical strength. Furthermore, a plastic substrate may be alternatively used, although it has not been common yet. Under such circumstances, the substrate is deformed by the pressure from inside and outside of the cell which is applied in bonding the TFT array substrate 10 together with the opposing substrate 30 for making a panel, which makes it more and more difficult to keep the gap between the both substrates even.
The present invention has been made in order to solve the above-described problems, and aims to provide a liquid crystal display device with excellent display quality.